Transport of oxygen in silicate glasses

Oxygen can be transported through silicate glasses and melts by water, oxygen and silicon monoxide molecules. These molecules dissolve and diffuse in the open glass structure, and oxygen atoms in them can exchange with the oxygen atoms in the silicate silicon-oxygen network. A large variety of measurements of water diffusion in silicates can be understood with the diffusion-reaction and exchange mechanisms of dissolved water molecules. The permeation measurements by Norton of oxygen through silica glass demonstrate the dissolution and diffusion of oxygen molecules. However, measurements of oxygen diffusion with O-18 isotope have given widely different results in different studies. The main reason for these differences is the influence of trace amounts of residual water in the oxygen gas and in the glasses. Thus these isotopic exchange measurements of oxygen diffusion in glasses can be understood and interpreted only when the amount of residual water is measured and controlled. Surface exchange between oxygen atoms in the gas phase and the glass is fast; the reason for a lower concentration ratio of oxygen isotope in the glass surface than in the gas phase is not a slow surface reaction but a slow internal exchange between oxygen atoms in dissolved molecules and network oxygen atoms. Diffusion of oxygen in silicates at temperatures above about 1300degreesC with high activation energy results from diffusion of dissolved SiO molecules, and is related to viscous flow. (C) 2004 Elsevier B.V. All rights reserved.